Process for upgrading methane to higher hydrocarbons

ABSTRACT

There is provided a process for the direct partial oxidation of methane with oxygen, whereby organic compounds comprising higher hydrocarbons are produced. The catalyst used in this reaction is an NiZSM-5 catalyst. This catalyst may be prepared by ion exchanging or impregnating a ZSM-5 catalyst with a suitable nickel salt such as nickel nitrate.

BACKGROUND

There is provided a process for the direct partial oxidation of methanewith oxygen, whereby organic compounds comprising liquid hydrocarbonsare produced. The catalyst used in this reaction is an NiZSM-5 catalyst.

Natural gas is an abundant fossil fuel resource. Recent estimates placesworldwide natural gas reserves at about 35×10¹⁴ standard cubic feet,corresponding to the energy equivalent of about 637 billion barrels ofoil.

The composition of natural gas at the wellhead varies but the majorhydrocarbon present is methane. For example the methane content ofnatural gas may vary within the range of from about 40 to 95 vol. %.Other constituents of natural gas may include ethane, propane, butanes,pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide,helium and nitrogen.

Natural gas is classified as dry or wet depending upon the amount ofcondensable hydrocarbons contained in it. Condensable hydrocarbonsgenerally comprise C₃ + hydrocarbons although some ethane may beincluded. Gas conditioning is required to alter the coxposition ofwellhead gas, processing facilities usually being located in or near theproduction fields. Conventional processing of wellhead natural gasyields processed natural gas containing at least a major amount ofmethane.

Processed natural gas, consisting essentially of methane, (typically85-95 volume percent) may be directly used as clean burning gaseous fuelfor industrial heat and power plants, for production of electricity, andto fire kilns in the cement and steel industries. It is also useful as achemicals feedstock, but large-scale use for this purpose is largelylimited to conversion to synthesis gas which in turn is used for themanufacture of methanol and ammonia. It is notable that for theforegoing uses no significant refining is required except for thoseinstances in which the wellhead-produced gas is sour, i.e., it containsexcessive amounts of hydrogen sulfide. Natural gas, however, hasessentially no value as a portable fuel at the present time. In liquidform, it has a density of 0.415 and a boiling point of minus 162° C.Thus, it is not readily adaptable to transport as a liquid except formarine transport in very large tanks with a low surface to volume ratio,in which unique instance the cargo itself acts as refrigerant, and thevolatilized methane serves as fuel to power the transport vessel.Large-scale use of natural gas often requires a sophisticated andextensive pipeline system.

A significant portion of the known natural gas reserves is associatedwith fields found in remote, difficulty accessible regions. For many ofthese remote fields, pipelining to bring the gas to potential users isnot economically feasible.

Indirectly converting methane to methanol by steam-reforming to producesynthesis gas as a first step, followed by catalytic synthesis ofmethanol is a well-known process. Aside from the technical complexityand the high cost of this two-step, indirect synthesis, the methanolproduct has a very limited market and does not appear to offer apractical way to utilize natural gas from remote fields. The Mobil OilProcess, developed in the last decade provides an effective means forcatalytically converting methanol to gasoline, e.g. as described in U.S.Pat. No. 3,894,107 to Butter et al. Although the market for gasoline ishuge compared with the market for methanol, and although this process iscurrently used in New Zealand, it is complex and its viability appearsto be limited to situations in which the cost for supplying an alternatesource of gasoline is exceptionally high. There evidently remains a needfor other ways to convert natural gas to higher valued and/or morereadily transportable products.

A reaction which has been extensively studied for many years is thedirect partial oxidation of methane to methanol. This route, involvingessentially the reaction of methane and gaseous oxygen according to thesimple equation

    CH.sub.4 +1/2O.sub.2 →CH.sub.3 OH

could theoretically produce methanol with no by-product. The homogeneousreaction of methane with oxygen to produce methanol occurs mostfavorably under high pressure (10 to 200 atm.), moderate temperatures,(350°-500° C.), and at relatively low oxygen concentration. Oxidation toformaldehyde and deep oxidation reactions are minimized under theseconditions. The mechanism of methanol formation is believed to involvethe methylperoxy radical (CH₃ OO.) which abstracts hydrogen frommethane. Unfortunately, the per pass yields have been limited. Thislimited yield has been rationalized as resulting from the low reactivityof the C--H bonds in methane vis-a-vis the higher reactivity of theprimary oxygenated product, methanol, which results in selectiveformation of the deep oxidation products CO and CO₂ when attempts aremade to increase conversion.

U.S. Pat. No. 4,618,732 to Gesser et al. describes an improvedhomogeneous process for converting natural gas to methanol. The allegedhigh selectivity for methanol is ascribed by the inventors to carefulpremixing of methane and oxygen and to eliminating reactor wall effectsby use of glass-lined reactors.

SUMMARY

There is provided a process for synthesizing a mixture of organiccompounds comprising higher hydrocarbons including C₅ + liquidhydrocarbons by the direct partial oxidation of methane, said processcomprising the steps of:

(i) contacting a mixture methane and oxygen with an NiZSM-5 catalystunder sufficient conversion conditions; and

(ii) recovering said mixture of organic compounds comprising higherhydrocarbons.

It will be understood that the expression, higher hydrocarbons, as usedherein, means hydrocarbons which have more carbon atoms (i.e. at least 2carbon atoms) than methane. It will be further understood that theexpression, C₅ + liquid hydrocarbons, as used herein, means liquidhydrocarbons which have at least 5 carbon atoms.

EMBODIMENTS

The NiZSM-5 catalyst comprises an intimate mixture of Ni and ZSM-5 in acatalytically active form. This catalyst may be prepared by ionexchanging or impregnating ZSM-5 with a suitable nickel salt. This ionexchange or impregnation may take place using either a bound or unbound(i.e., binder free) form of ZSM-5. An example of a bound form of ZSM-5is an extrudate of ZSM-5 and an alumina binder.

The nickel salt which is ion exchanged or impregnated with ZSM-5 may bea salt which is capable of being converted to nickel oxide uponcalcination, especially in the presence of oxygen. An example of such anickel salt is nickel nitrate.

In the practice of the present invention, it is preferred to use a dualflow system, i.e., a system in which the natural gas and the oxygen orair are kept separate until mixed just prior to being introduced intothe reactor. However, if desired, the oxygen and natural gas may bepremixed and stored together prior to the reaction. The preferred dualflow system minimizes the risk of fire or explosion. In the dual flowsystem, the amount of oxygen flow may be controlled so as to prepare areaction mixture that contains 2 to 20 percent by volume, morepreferably 3 to 15 percent of oxygen. Air may be used instead of oxygenwithout affecting the reaction.

The temperature in the reaction zone may be from about 300° C. to 500°C., and preferably about 350° C. to 475° C. In the preferred mode ofoperation, the reactor temperature is increased until substantially allof the oxygen is consumed by the reaction, i.e., greater than about 90percent 0₂ consumption, and then it is held at about that temperatureuntil further adjustment is required.

The gas hourly space velocity (GHSV) on zeolite may be within the rangeof about 100 to 100,000 hr ⁻¹, preferably about 1,000 to 10,000 hr ⁻¹,and most preferably about 2,000 to 8,000 hr ⁻¹.

COMPARATIVE EXAMPLE A

An (H)ZSM-5 catalyst having a silica to alumina ratio of 70:1 was mixedwith alumina (65% zeolite, 35% binder) and formed into extrudate. Theextrudate, ground to 20-40 mesh, (8.0 cc) was mixed with an equal volumeof sand and loaded into the reactor's 9/16 inch I.D. pyrex glass liner.

Feed mixtures were prepared from ultra high purity methane and C.P.grade oxygen supplied by Matheson, and the feed was passed over (H)ZSM-5catalyst. Reaction conditions and the results of the experiments areincluded in Table I.

COMPARATIVE EXAMPLE B

PtZSM-5 was prepared by ion-exchanging 10 g of the Comparative Example Acatalyst with 2.7 g of Pt (NH₃)₂ (NO₃)₂ in 200 g H₂ O for 24 h at 25° C.The resultant catalyst was washed with water, dried and air-calcined.

This PtZSM-5 catalyst was contacted with a mixture of methane and oxygenin the manner described in Comparative Example A. Reaction conditionsand results are included in Table I.

COMPARATIVE EXAMPLE C

CrZSM-5 was prepared by ion-exchanging 15 g of the Comparative Example Acatalyst with 4.2 g Cr(NO ₃)₃.9H₂ O in 200 g H₂ O for 24 h at 25° C. Theresultant catalyst was washed with water, dried and air-calcined.

This CrZSM-5 catalyst was contacted with a mixture of methane and oxygenin the manner described in Comparative Example A. Reaction conditionsand results are included in Table I.

EXAMPLE 1

NiZSM-5 was prepared by ion-exchanging 15 g of the Comparative Example Acatalyst with 3.0 g Ni(NO₃)₂.6H₂ O in 200 g H₂ O for 20 h at 25° C. Theresultant catalyst was washed with water, dried and air-calcined.

This NiZSM-5 catalyst was contacted with a mixture of methane and oxygenin the manner described in Comparative Example A. Reaction conditionsand results are included in Table I.

EXAMPLE 2

The catalyst loading used in this Example was the same used inExample 1. This catalyst loading was contacted with mixture of methaneand oxygen and product collection began after 8 hours time on stream.Reaction conditions and results are included in Table I.

                                      TABLE 1                                     __________________________________________________________________________    Catalytic Data and Results                                                    Example or                                                                    Comparative Example                                                                         1    2    A    B    C                                           __________________________________________________________________________    Catalyst      NiZSM-5                                                                            NiZSM-5                                                                            HZSM-5                                                                             PtZSM-5                                                                            CrZSM-5                                     Time on Stream                                                                              fresh                                                                              8 hrs.                                                                             fresh                                                                              fresh                                                                              fresh                                       Pressure, psig                                                                              960  960  960  960  960                                         Temp., °C.                                                                           465  465  450  450  440                                         GHSV (zeolite), hr .sup.-1                                                                  4600 4600 4600 4600 4600                                        % O.sub.2 in feed                                                                           7.3  7.3  6.9  7.0  6.8                                         CH.sub.4, conv., %                                                                          5.2  5.7  5.2  4.7  5.2                                         Selectivities, %                                                              CO.sub.x      83.8 84.0 83.3 97.2 99.2                                        CH.sub.3 OH   6.3  4.5  16.7 1.8  0.8                                         Other aq. phase oxygenates                                                                  0.6  1.0  --   --   --                                          C.sub.2 -C.sub.4                                                                            5.0  4.5  --   1.0  --                                          C.sub.5 +     4.3  6.0  --   --   --                                          __________________________________________________________________________

As the data in Table 1 clearly show, addition of Ni to the ZSM-5catalyst improved the C₂ -C₄ and C₅ + selectivities from methane atsimilar conversions. C₅ + liquids are not produced from methane/oxygenfeeds over unmodified ZSM-5, CrZSM-5 or PtZSM-5 under similarconditions.

We claim:
 1. A process for synthesizing a mixture of organic compoundscomprising higher hydrocarbons including C₅ + liquid hydrocarbons by thedirect partial oxidation of methane, said process comprising the stepsof:(i) contacting a mixture of methane and oxygen with a NiZSM-5catalyst under sufficient conversion conditions including a temperatureof from about 350° C. to about 475° C. and a pressure of from about 150psig to about 3000 psig; and (ii) recovering said mixture of organiccompounds comprising higher hydrocarbons.
 2. A process according toclaim 1, wherein said mixture of methane and oxygen contains from about3 to about 20 percent by volume of 0₂.